What exactly is the benefit to importing personnel who will work for lower wages and draining talent from other nations when we have 300MM+ local people from wh

What exactly is the benefit to importing personnel who will work for lower wages and draining talent from other nations when we have 300MM+ local people from which we could train medical workers? I fundamentally don't understand why people feel that they are entitled to a job in the US, that we somehow owe people from other nations a chance to immigrate, when we have entire swaths here in the country that have been effectively left behind by the economy (inner cities and rural america, especially Appalachia). Maybe we should focus more on training our existing citizens out of poverty and into skilled labor, rather than brain-draining the rest of the world.